A large number of industries require precise and accurate measuring for a number of applications such as production, manufacturing, and process control. In many such applications, measurement errors on the order of even one ten-thousandth of an inch can be critical. Often, especially when measuring the dimensions of large objects or a long distance between two objects, special equipment and/or instruments are used to achieve the necessary precision and accuracy for a particular application. Instruments such as laser trackers are particularly well suited for such applications because laser trackers are capable of providing extreme precision and accuracy, even when measuring the dimensions of large objects.
As is the case with calibration (or verifying proper calibration) of virtually any measuring instrument, checking calibration of a laser tracker is typically accomplished by measuring an object (such as a length reference artifact) of a standard, known length and confirming that the instrument measures the appropriate length. In particular, such artifacts are used to verify whether a laser tracker instrument is yielding trustworthy results (i.e. demonstrating that it is properly calibrated) or used during a measurement job to establish whether anything has gone wrong during the course of the job. For example, a user of the instrument will measure the artifact at the beginning, middle, and end of a job. If the user obtains the same length measurement each time, the user will have a degree of comfort that the tracker has maintained a proper adjustment and/or position during the measurement.
An acceptable method used to verify the proper calibration of a laser tracker is a length measurement system test. A length measurement system test involves several steps. First, two or more measuring points are located and oriented relative to a laser tracker. The measuring points are displaced a known distance from each other; the known distance being a reference length. Next, the laser tracker measures the distance between each measuring point; the measured distance being a measured length. Finally, the measured length is compared with the reference length so as to evaluate the performance of the laser tracker. Because a laser tracker may perform differently depending on the position and orientation of measuring points relative to the laser tracker, the above steps are repeated with the measuring points at various locations and orientations to exercise the various error sources within the tracker.
Prior to performing a length measurement system test, the reference length must be established. One method of establishing a reference length is described in Section D-3 of ASME B89.4.19-2006. Using this method, the measuring points are aligned with the laser tracker so that the distance between the measuring points may be measured with the more accurate ranging system of the tracker (such as an interferometer or Absolute Distance Meter system). Another method of establishing a reference length is to use a calibrated artifact.
A calibrated artifact includes a measuring point on or near each end of the artifact. Traditionally, artifacts were calibrated at a specific temperature range and nominal thermal expansion corrections are provided to establish the reference length between the measuring points. More recently, several length reference systems have been developed that include structural arrangements that are designed to counteract length increases caused by thermal expansion. For example, U.S. application Ser. No. 13/431,188, now U.S. Pat. No. 8,479,406, the entire disclosure of which is incorporated herein by reference, discloses a length reference bar system and method that compensates for thermal expansion and is capable of being calibrated using the method described in Section D-3 of ASME B89.4.19-2006. Other length reference systems disclosed in U.S. Pat. Nos. 6,505,495, 6,836,323, and 7,188,428, the entire disclosures of which are incorporated herein by reference, have also been designed to counteract length increases caused by thermal expansion. Still other length reference systems are fabricated from materials having low coefficients of thermal expansion, such as carbon composite and/or high-grade invar, to further reduce the artifact's sensitivity to temperature gradients.
When using an artifact to perform a length measurement system test, the artifact is positioned and oriented so as to move the measuring points to various locations and orientations relative to the laser tracker. For example, the artifact may be oriented vertically for a first test, horizontally for a second test, and diagonally for a third test. In each orientation, the measuring points may be positioned symmetrically or asymmetrically relative to the laser tracker.
Precise and accurate movement of an artifact to a specific position and orientation is time consuming and difficult. Consequently, positioning repeatability is also time consuming and difficult. Once in position, maintaining the position and orientation of the artifact can be difficult. A fixture may solve some of these problems, but a fixture also creates additional uncertainty with the accuracy of the reference length.
To ensure that a length measurement system test is as accurate as possible, the reference length must be as close as possible to the actual distance between the measuring points at the time the measured length is obtained. Unfortunately, several factors, such as “fixturing effects,” create uncertainty as to the accuracy of a reference length “Fixturing effects” may include, but are not limited to, gravity effects, loading effects, and mounting constraint effects. “Fixturing effects” may be influenced by factors such as the straightness and/or stiffness of an artifact, the type, quantity, and/or location of mounting fixtures, the location of targets relative to the mounting fixtures, potential vibration, and/or the accuracy of an alignment. Additionally, uncertainty of the artifact temperature and uncertainty of the coefficient of thermal expansion of the artifact material create uncertainty with the thermal expansion correction values (if used).
Fixturing effects may vary with temperature and/or with the orientation of the artifact. Accordingly, fixturing effects may be difficult to detect and/or to predict. For instance, an artifact at room temperature and situated in a vertical orientation may experience negligible fixturing effects while an artifact at twenty degrees above room temperature and situated in a horizontal orientation may experience various fixturing effects such as thermal expansion restraint and/or cantilever bending. Thermal expansion restraint creates additional uncertainty with the thermal expansion correction values. Cantilever bending creates additional uncertainty with the calibrated length of the artifact at various orientations.
Therefore, it is desirable to provide a reference length system and method that quickly moves measuring points into precise and accurate locations and orientations relative to a laser tracker, thereby allowing for positioning repeatability, that maintains an accurate reference length, that is easy to manufacture, and that is simple to use. Furthermore, it is desirable to provide a system and method for quickly and easily determining a reliable reference length between measuring points.